That is most likely the result of an incorrect subscription form. The value for the "action" tag for unsubscribing must be "unsubscribe2", not "unsubscribe." This is a common mistake, and there is a reason for this requirement, even though I've long forgotten what exactly that reason is...

You should probably download the form generator plugin if you're going to be making a number of subscription forms, as it will create the HTML correctly.